Over the past decade, numerous studies conducted around the world have noted that women who work the night shift exhibit an increased risk for breast cancer. But the rate of increase, suspected to be due to hormonal changes associated with light exposure, was thought to be relatively low.

Now the most comprehensive investigation to date of possible links between outdoor light at night and breast cancer risk suggests that exposure to such light can increase a woman’s chances of developing breast cancer by up to 14 percent. The findings, which are presented in the journal Environmental Health Perspectives, particularly apply to women who are premenopausal and who are current or past smokers.

“The growing research on light at night and breast cancer is based on data suggesting that exposure to light during nighttime hours suppresses nocturnal secretion of melatonin and disrupts circadian patterns and sleep,” lead author Peter James, an assistant professor at Harvard Medical School’s Department of Population Medicine at the Harvard Pilgrim Health Care Institute, told Seeker.

Melatonin is a hormone produced by a small gland in the brain. It helps to control sleep and wake cycles. The hormone is also found in various foods, and can be purchased as a supplement.

James explained that the circadian clock regulates the cell cycle, and that “disruption of circadian patterns could be associated with abnormal cell divisions that occur in cancer. Indeed, animal studies have shown that mice subjected to an inverted light cycle experience higher levels of mammary tumors.”

Composite of satellite images showing what the US looks like from space at night. |
NASA/GSFC

In this case, the study looked at data from nearly 110,000 women who enrolled in the Nurses’ Health Study II from 1989–2013, one of the largest studies of risk factors for serious chronic diseases in women.

James and his team linked data from satellite images of Earth taken at night to the residential address of each study participant. He and his team also considered the influence of night shift work, and factored in detailed information on a variety of health and socioeconomic factors among the participants.

The researchers identified 3,549 incidents of breast cancer among the women in the study. An association between exposure to light at night and breast cancer was seen, not only in premenopausal women and current and past smokers, but also among those women who worked night shifts.

Night nurses and others who work in the evenings and early morning hours have no choice but to be exposed to ample artificial light. Even those of us who do not work the graveyard shift might often look at our televisions, cell phones, laptops, and other artificially lit devices in the evenings.

Street lamps, neon business signs, and other artificial outdoor lighting surround many of our homes. All of this begs the question: Can even short-term, yet regular, exposure to artificial light at night also lead to health problems?

“Although our study did not examine this question, previous studies have shown that exposure to light from electronic devices may suppress melatonin and shift the circadian clock,” James said. “I do not think we know enough to say exactly how frequent or chronic exposure to this type of light would have to be to influence health outcomes.”

Women aren’t the only ones at potential risk, either.

James said that other prior research has suggested that night shift work and exposure to light at night could be linked to several types of cancer, including prostate cancer.

Increased incidences of diabetes, heart disease and obesity related to the same factors have also been reported.

“So, the risks of exposure to light at night may not be confined to women,” James said, “but again, this area requires a great deal more research before we can make definitive recommendations.”